1. Field of the Invention
The present invention is related to a photo-luminescent device for producing visible light upon excitation, or for producing visual images if the excitation light is modulated. More particularly, the present invention relates to the packaging of a photo-luminescent device to improve the effective use of the photo-luminescent material. The present invention also relates generally to visual display systems, and more particularly to visual display systems utilizing photo-luminescent materials in the image screen or in the illumination source of the display.
2. Description of the Related Art
Phosphors, as a type of luminescent material, have been used in the devices such as cathode-ray-tubes, x-ray screens, photo-luminescent displays, and lighting or illumination products such as florescent lamps and phosphor-converted white light-emitting diodes (LEDs). Phosphors can absorb the energy from the excitation source and convert it to visible light or other wavelengths. Photo-luminescent phosphors are the type of phosphors that can convert the radiant energy of the light from the excitation source to a different wavelength. For visual displays, the emission of the phosphors is in the visible range to human eyes, namely, in the 400 nm-750 nm region. The excitation source for these phosphors has a wavelength typically in a band with higher energy, i.e. from UV (180 nm-360 nm) to blue light (360 nm-480 nm) region.
Phosphors, upon absorbing the excitation energy, will generate heat because of the conversion loss. The emitted light from the phosphor radiate isotropically into the space, half forward and half backward about the phosphor plane. Phosphors that can be excited by blue light can also be excited not only by the blue light from the excitation source, but also the blue light in typical natural day light. The self-heating of phosphor, if not managed properly, can heat up the phosphor material, resulting in reduction in emission efficiency, and in extreme cases, thermal quenching of the light emission. Intense heating can also result in physical and chemical degradation of phosphor material, reducing the lifetime of the phosphor and performance of the device. The light emitted from the phosphor but backward toward the excitation source will not be usable if not re-directed toward the forward direction, resulting in loss of half of the light. It is critical in phosphor packaging design to utilize both backward and forward emission from the phosphor for maximal optical efficiency.
Conventional phosphor screens, such as the faceplate of a cathode ray tube (CRT) or the phosphor plate used for x-ray imaging, comprise a thin layer of highly reflective aluminum coating on the phosphor. This aluminum coating allows the electron beam or x-ray to penetrate through to excite the phosphor, but reflects the backward emission from the phosphor toward the forward direction, improving the effective optical efficiency of the phosphor. In addition, the aluminum coating helps dissipate the heat generated in the phosphor layer, because of the high thermal conductivity of aluminum.
For UV excited phosphors, such as phosphors used in the fluorescent lamps, the normal ambient light will not be able to provide excitation sufficient to produce significant phosphor emission. However, blue-excited phosphors, can be excited not only from the light from the excitation source, but also by the typical visible light in ambient. For lighting applications, this does not seem to be a problem. For display applications, the phosphor should be excited only by the controlled light from the excitation source. In image forming screen applications, the phosphor screens using blue excitation light but without consideration of controlling natural ambient light has a low contrast ratio because of the phosphor emission caused by the ambient light. This is not acceptable for high fidelity display systems.